K. Csach

New features of the low temperature ductile shear failure observed in bulk amorphous alloys

Fractographic studies of ductile shear failure under the uniaxial compression for rod–like samples of the Zr41.2Ti13.8Ni10Cu12.5Be22.5 and Cu50Zr35Ti8Hf5Ni2 bulk amorphous alloys at temperatures 300 and 77 K are presented. The mechanisms of shear deformation and failure appeared to have characteristics in common with other amorphous alloys prepared in the form of thin ribbons. However, there were a number of new fractographic features observed due to the bulk character of the samples and to the large supercooled liquid region of these alloys.

Structure of water-based ferrofluids with sodium oleate and polyethylene glycol stabilization by small-angle neutron scattering: contrast-variation experiments

Contrast variation in small-angle neutron scattering (SANS) experiments is used to compare the structures of a water-based ferrofluid, where magnetite nanoparticles are stabilized by sodium oleate, and its mixture with biocompatible polyethylene glycol, PEG. The basic functions approach is applied, which takes into account the effects of polydispersity and magnetic scattering. Different types of stable aggregates of colloidal particles are revealed in both fluids. The addition of PEG results in a reorganization of the structure of the aggregates: the initial comparatively small and compact aggregates (about 40 nm in size) are replaced by large (more than 120 nm in size) fractal-type structures. It is postulated that these large structures are composed of single magnetite particles coated with PEG, which replaces sodium oleate. Micelle formation involving free sodium oleate is observed in both fluids. The structures of the fluids remain unchanged with increasing temperature up to 343 K. New and specific possibilities of SANS contrast variation with respect to multicomponent systems with different aggregates are considered.

Anelastic deformation processes in metallic glasses and activation energy spectrum model

Abstract The isothermal kinetics of anelastic deformation below the glass transition temperature (so-called ‘stress induced ordering’ or ‘creep recovery’ deformation) was investigated in Ni-Si-B metallic glass. The relaxation time spectrum model and two recently developed methods for its calculation from the isothermal experimental data sets seem to be powerful tool for the study of anelastic deformation processes in disordered systems. Reliability of the used calculation methods (the first is based on the Fourier transformation technique and the second on the non-linear regression) is expressed in the fact that both provide qualitatively the same result (without prior assumptions concerning the form or nature of the spectrum): creep recovery relaxation spectrum consists of three relatively separated peaks with the well defined characteristic relaxation times. The influence of (i) loading time, (ii) the structural relaxation and (iii) unloading temperature on the spectrum is demonstrated and discussed. Using the assumption that different peaks in creep recovery spectra represent different type of elementary deformation processes (or different type of deformation defects) detected during the creep recovery, discussion concerning the eventual deformation defects will be actualized.

Possible local superplasticity of amorphous metallic alloys in the catastrophic shear band under low temperature ductile shear failure

The vein pattern (VP) formation on the failure surfaces is a characteristic feature of the low temperature ductile shear failure of amorphous alloys. Argon and Salama proposed the meniscus instability in the thin liquid-like layer (layer with a low viscosity) as a mechanism of the VP formation. The real instant plasticity of the material in the liquid-like layer can be estimated from the plastic deformation localized in single veins. In this work the authors have observed and estimated the magnitude of the plastic deformation in these protruding veins. This estimation has been realized by the fractographic analysis of fractured surfaces for various amorphous alloys. Failure surfaces after the low temperature (0.5--300 K) shear failure were observed using a TESLA BS-300 scanning electron microscope for various amorphous alloys deformed under uniaxial tension (Fe{sub 84}B{sub 16} and Fe{sub 80}B{sub 20} amorphous ribbons with the cross sections of 6.0 {times} 0.03 mm{sup 2}, Pd{sub 84.5}Si{sub 15.5} amorphous ribbon with cross section of 2 {times} 0.03 mm{sup 2}) and under uniaxial compression (Cu{sub 50}Zr{sub 35}Ti{sub 8}Hf{sub 5}Ni{sub 2} amorphous rod with 3 mm diameter).

Non-Newtonian plastic flow of a Ni-Si-B metallic glass at low stresses

The problem of the rheological behavior of metallic glasses (MGs) is quite important both from theoretical and practical viewpoints. Early experiments carried out on MGs at temperatures T > 300 K using low shear stress levels revealed plastic flow to be Newtonian while measurements at relative high shear stresses (more than 200 to 400 MPa, depending on temperature, thermal prehistory of samples and chemical composition) indicated a non-linear behavior with 1 < m < 12. Numerous investigations performed later both on as-cast and relaxed MGs of various chemical compositions using a number of testing methods (tensile creep, tensile and bend stress relaxation) showed that a transition from Newtonian behavior at low stresses to a non-linear flow at high stresses was observed. At present, such a situation is considered to be generally accepted. The authors performed precise creep measurements of a Ni-Si-B metallic glass. The results obtained indicate that plastic flow in this case at low tensile stress (12 {le} {sigma} {le} 307 MPa) is clearly non-Newtonian and, consequently, the viscosity is stress dependent.

Microstructure and Phase Formation in a Rapidly Solidified Laser-Deposited Ni-Cr-B-Si-C Hardfacing Alloy

In this study, microstructural evolutions and phase selection phenomena during laser deposition of a hardfacing Ni-Cr-B-Si-C alloy at different processing conditions are experimentally investigated. The results show that even minor variations in the thermal conditions during solidification can modify the type and morphology of the phases. Higher undercoolings obtained at faster cooling rates suppressed the primary borides and encouraged floret-shape mixtures of Ni and Cr5B3via a metastable reaction. Variations in the boride phases are discussed in terms of nucleation- and growth-controlled phase selection mechanisms. These selection processes also influenced the nature and proportion of the Ni-B-Si eutectics by changing the amount of the boron available for the final eutectic reactions. The results of this work emphasize the importance of controlling the cooling rate during deposition of these industrially important alloys using laser beam or other rapid solidification techniques.

Direct spectrum analysis of anelastic deformation response during structural relaxation of amorphous metals

Direct relaxation time spectrum analysis method has been successfully used to observe and to study changes in the kinetics of isothermal anelastic deformation response of soft magnetic metallic glass Fe/sub 40/Ni/sub 40/B/sub 20/ during structural relaxation. Computed relaxation time spectra contain three or four quite well separated peaks. >

Activation energy distribution in nanocrystallization kinetics of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

Nanocrystalline phase produced by suitable annealing of iron-based amorphous alloys known as Finemets shows extremely good soft magnetic properties. It is desirable to understand the mechanisms by which these materials crystallize in order to prepare relative stable materials for prospective applications. The presence of crystalline grains in the nanometer range in Finemet alloys is considered to be a consequence of processes of compositional and structural ordering. Therefore, the electrical resistivity variations occurring in amorphous alloys during the isothermal annealing are usually exploited to obtain information on the kinetics of the nanocrystallization process. Assuming inhomogeneities in the nanoscale range the authors applied the idea of Krueger and Woldt on isothermal resistivity transformation curves during the nanocrystallization of Fe{sub 73.5}Cu{sub 1}Nb{sub 3}Si{sub 13.5}B{sub 9}. It will be presented that characterization by a rather simple distribution of activation energies is possible.

Foam coating on aluminum alloy with laser cladding

This article concentrates on the creation of a foam layer on an Al–Si substrate with laser technology. The cladding of Al–Si powder in the front of a laser track has been separated from the side injection of mixture of Al–Si/TiH2 powder (foaming agent), which allows for fine tuning of the main processing parameters, i.e., melting temperature and laser processing time. A wide operational parameter window has been found and the formation of a porous layer has been observed for all combinations of Nd:YAG laser power in the range of 1.0–1.5 kW and for scanning speeds between 2.5 and 5.8 mm/s. Microstructural examinations were focused on the study of the relationship between process parameters and foam characteristics (porosity and bubbles size distribution). A tensile test confirmed that due to the relatively low porosity the tensile behavior of the foam layer is closer to the behavior of bulk aluminum than conventional closed cell aluminum foams.

FAILURE CRACK ORIENTATION AT DUCTILE SHEAR FRACTURE OF FE80-XNIXB20 METALLIC-GLASS RIBBONS

AbstractOrientation of failure cracks of oblique mode at ductile shear fracture are measured for Fe80−xNixB20 (x=10, 20, 30, 40, 50, 60) metallic glass ribbons (MGR) uniaxially extended with strain rate